Diodes, Dudes!

[Steve L.]

I'm curious about diodes in the tow/trailer charging circuit. If electrical stuff is boring to you, please pass on by. We've already established I'm a geek so let me be. Today I'm a militant geek. I just want to learn something.

I'm imagining a trailer with a low battery. I decide to hook the trailer up to the tow vehicle.

First I'm imagining the wiring as looking like shown in my simple sketch. (In my drawing, I just threw a light bulb into the trailer circuit. It's not even turned on.)
The tow vehicle is hooked up to the trailer. When the engine is running, Mr. V. Oltage Regulator is eyeballin' the tow battery. He's got one hand on the alternator dialing in enough amps to keep that battery pretty much filled. Note that the tow vehicle battery doesn't have much capacity. Its 13.6 volts tall, but it drains pretty quickly.

And keep in mind that the regulator doesn't have much to work with. You're not pedaling that tow vehicle very fast. At a fast idle, or even highway cruising speeds your alternator is probably only putting out maybe half its capacity and you're using up most of it just running the tow vehicle let alone recharging anything.

By the way, I picked 13.6v as full because I was too lazy to differentiate between a full battery and a charging battery.

The way I have it drawn, the voltage in the tow vehicle battery (13.6) is higher than the depleted trailer battery (12.0). Amps are going to flow from the tow battery back to the trailer battery. If the engine is running, then likely Mr. Regulator will bump up his amperage level just enough to make up for whatever is being actively consumed by the trailer.

As long as there is no additional drain on the trailer battery, eventually, slowly, bit by bit, the trailer battery will refill. Not quickly because the regulator isn't seeing the problem directly, but rather filtered through the tow vehicle's battery.

If you're putting a significant electric drain on the trailer battery, I doubt the alternator will ever recharge the trailer battery. The regulator is just replacing what is lost to the tow battery, probably only as fast as it goes out. No faster.

Turn off the engine and the levels of the two batteries will seek to level out pretty much evenly. There's nothing to stop the tow vehicle battery from draining into the trailer battery. Further, because the trailer battery has so many more amp-hours in it than the tow vehicle battery, the resultant voltage in both batteries will be closer to 12.0 volts than it will be to 13.6 volts.

Many people have this wiring and they handle it by unplugging the trailer when they stop for anything longer than a few minutes.

[Steve L.]

My Explorer has the positive line to the trailer on a relay tied in to the ignition key. Turn off the ignition, then the relay opens and no current flows from the tow to the trailer. My Explorer doesn't charge the trailer battery any more or less than the first example. However, with the engine off the trailer won't drain the tow battery.

[Steve L.]

Some people install diodes to more or less duplicate what the relay does for me. I wondered where the diode goes to keep the tow battery from draining. Putting it like this doesn't do any good. Put the diode there and all it does is keep the trailer battery from charging the tow battery.

Not necessarily a trivial issue. The trailer might be plugged into shore power, the converter putting 13.6 vdc into the trailer battery and I might have the tow vehicle connected but not running and current might try to recharge the tow vehicle battery in this situation. (But not on the Explorer if I have the key off and the relay is open.)

[Steve L.]

This is all I can come up with. This location allows the alternator to provide current to the trailer and the tow battery. How much the regulator tells the alternator to send must be some sort of "average" of the tow and the trailer voltages. Plus, what arrives at the trailer battery is reduced a significant amount by the electrical resistance in the long wiring run back to the trailer battery.

Turn off the engine, which shuts off the alternator, and the diode keeps juice from going from the tow battery to the trailer battery. This means you'd have to interrupt the charge line from the alternator to the tow vehicle battery to install the diode, then splice in the trailer charge line "upstream" of the diode.

So… For those of you who have done this, is this how you wire an aftermarket diode?

[Morgan]

Actually, Steve, the diode shown in your third post prevents the trailer from recharging the tow vehicle battery.

The diode in the fourth post prevents the tow vehicle battery from discharging to the trailer.

There is a problem with using a diode in either application, however, and that is the voltage drop across the diode. A silicon diode drops about 0.6 volts when current is flowing in the forward direction. If Mr V is watching the wrong side of the diode, he'll charge when he shouldn't or not charge when he should because he sees the battery voltage minus 0.6 volts. In your example, he sees 13.0 volts instead of the 13.6 volts at the battery. Therefore he'll say, "lets get this dude charged up."

[Byron Kinnaman]

The configuration in your last post is pretty much the way with a diode isolator. Here's a bit more information.

http://www.bcae1.com/battiso.htm

[Brian B-P]

To continue what Morgan has explained, one diode can keep the trailer from draining the tow battery, and another can keep the tow vehicle from draining the trailer battery, but you need two diodes to do the whole job. I believe that the typical diode isolator, such as the one linked by Byron, has the two diodes, with one common connection, and thus has those visible three high-current terminals, with a configuration as shown in the this drawing crudely adapted from steve's last one:




To address the problem of control which Morgan mentions, some isolators have that fourth terminal which may be used by some alternators to "sense" the voltage on the battery side of the diodes, so that the desired full battery voltage is reached.

I use the relay approach in my Sienna to supply my Boler, mostly due to the voltage drop issue, and to avoid interfering with factory wiring with an intervening diode.

[Byron Kinnaman]

I prefer the relay approach myself.

[Bill Abbay]

Steve,

Byron's link hits the nail on the head. It doesn't show the two diodes necessary to make this work. On the diagram shown there, they sit "back to back" astraddle the junction with the alternator. In other words, one diode "points" away from the juction to the tow battery, the other diode, on the other side of the junction, "points" toward the RV battery. Thus, neither battery sees the other and the alternator only deals with the current draw, whatever the source, at the junction.

Morgan's point about the voltage drop across the diode is important. Isolators for some GMC electrical systems have an auxillary line to the sense input of the alternator that effectively fools the alternator into putting out a higher voltage than 13.6 ... namely 14.2 volts or so. (There's nothing to be done about this voltage drop. It's built into semiconductor physics and is pretty much the same for any diode, large or small.)

I've only fiddled with GMC so don't know how, or if, isolators for other makes compensate for the voltage drop. Given that 1/2 volt represents a significant amount of capacity in the discharge curve of a deep cycle battery, it needs to be dealt with.

IMHO, without compensation for the diode voltage drop, the ignition relay approach is better, despite not being as elegant as diodes.

[Bigfoot Mike]

I have installed a hybrid system of what you have here. In my system I use a relay to turn power on & off to the trailer connection.

At the trailer I have a “Y” connection inside the closet where the power enters the RV. One leg goes to a diode > battery charging. The other leg of the “Y” connection goes directly to the refrigerator via a 10 gauge wire.

When the Tahoe is running it is providing power to the refrigerator. If I have the refrigerator on 12 volts, it operates it. When we are towing the RV battery is being charged to some extent.

Now when the Tahoe is turned off (ie at a restaurant) there is no bleed back from the Tahoe because of the relay, and NO bleed back from the RV battery because of the diode.

This has worked out very well on long trips keeping the refrigerator cold and not having to remember to unplug or turn anything off when we stop. We also don’t need to run the refrigerator on propane to keep it cold.

[Darwin Maring]

Go to JC Whitney and purchase a battery issolator. It will charge the camper battery and prevent your vehicle battery from discharging should you keep things connected up whild camping.

[Byron Kinnaman]

Go to JC Whitney and purchase a battery issolator. It will charge the camper battery and prevent your vehicle battery from discharging should you keep things connected up whild camping.

An isolator is just a couple of diodes encased in potting and terminals to make connections to. The working part is still diodes with .5 to .7 volt drop across each diode.

[Bigfoot Mike]

An isolator is just a couple of diodes encased in potting and terminals to make connections to. The working part is still diodes with .5 to .7 volt drop across each diode.

and, and, and, ...
<blockquote>they have big fins to get rid of all the heat.</blockquote>

[Bill Abbay]

I can only speak to my experience with a '93 GMC 2500. I bought it used and it had a Surepower isolator mounted in the engine compartment on left fender. It was not hooked up. Thinking this was cool, as I bought the truck to tow with, I endeavored to hook it up. After doing some research, I learned I needed a special adapter for the alternator sensing connection AND I would have to physically modify the wiring at the connector AND the diagrams included with the adapter kit didn't match anything I had. This went from a trivially simple, in principle, idea to something more, in practice.

I passed.

The screw terminals on the isolator proved handy to hook wires up to the straight-through connection to my 7-pin trailer connection at the back. Not real sophisticated but worked fine for several years.

I'm sure there's a large population of diode isolator users out there. My old tow vehicle and I just aren't members.

[Loren G. Hedahl]

In a ideal situation, the tow vehicle would be equipped with dual alternators; one for the tow vehicle and another for the trailer. The one for the trailer would have a large gauge charging wire and a separate sense wire. The use of a separate sense wire would eliminate sensing the 'voltage drop' in the charge line during high current charging to give a more accurage reading of the voltage at the trailer battery.

Most vehicles aren't available with dual alternators (however I faintly remember that a neighbor's Ford PowerStroke did), and I like most live in a less than an ideal world. I tend to go with the Keep It Simple, Stupid! idea unless there is a good reason not to.

So, what I do is just wire the two batteries in parallel. The resistance in the long charging wire to the trailer causes the tow vehicle battery to be charged first, which is great because it is a starting battery vs a deep discharge battery. For short stops I don't disconnect the plug to the trailer. But for longer, especially for overnight in cold situations, I disconnect.

I also have a small 2hp Honda from a lawn edger hooked up to a GM alternator, just in case!

[Gina D.]

*sniff sniff. Does someone smell geek in here?*

I am with Byron AND Loren. A relay seems the best way to go if you have to have something, BUT.. I just unplug the trailer. Thats pretty much fool proof, except for the fool that forgets to unplug

I did for one nite on my last trip with no appearant badness. I did not run the Elements battery down, and since I was pluugged into AC, my charger was a goin'. Probably huffing and puffing at keeping 2 batteries up to speed.

Most of my routines and habits when camped would prevent too much damage when boondocking. My routine after leveling is to switch the frige before anything else is started. All else is low draw. If staying more than one nite, I always unhook anyway.

As usual, I also have the back ups. My generator can charge the car battery, or that's what Good Sams is for

[Gina D.]

oh yeah. I am not a dude.

Get over it.

[Byron Kinnaman]

oh yeah. I am not a dude.

Get over it.

I guess he should have added dudettes to subject line.

[Per Walthinsen]

I vote for the simple relay. The audible click when you start the car reminds you that the thing is working, and you can't make a wrong move with it. Senility proof.

[Steve L.]

As long as it's dude-ette, not dud-ette.

I've read about the isolators and I wasn't unduly impressed. Reading the responses pretty much reinforces my first impressions.

I'm lucky enough to have the relay as part of the Ford towing package so I'm good to go.

I also wanted to reexamine the whole "charging the trailer battery from the tow vehicle" topic. I'm still not persuaded that raw amp-hours are returned to the trailer battery in any volume. Some capacity may be returned but not much.

Loren mentions the dual alternator route. I found that approach on the sites dedicated to the thumpin' radio modifying sites. They often turn to a second alternator and a second battery, usually deep cycle, to keep those menaces to quiet contemplation up and annoying. The approach would work for us as well but I don't think it's particularly cost effective what with chargers and generators and solar panels (and windmill, solar panel, starlight collecting inventions discussed elsewhere).